2-Hydroxynaphthalene-3-carboxylic acid is a compound important as an intermediate for various dyes and pigments, especially for the coupling component (grounder) of azoic dyes and the coupling component of azo dyes and azo lakes. The production of dyes and pigments of good quality require pure 2-hydroxynaphthalene-3-carboxylic acid that contains no impurities, particularly .beta.-naphthol or tar-like by-product.
The traditional process for the production of 2-hydroxynaphthalene-3-carboxylic acid uses the Kolbe-Schmitt reaction which is a solid-gas phase reaction between an alkali metal salt of .beta.-naphthol and carbon dioxide (Ullmanns Encyklopadie der Technischen Chemie, vol. 12, p. 606). However, the Kolbe-Schmitt reaction is difficult to control and constant yield is hard to achieve: the reaction involves great loss of .beta.-naphthol because it is a thermally heterogeneous reaction: the reaction takes place at least 40 to 50 hours to complete dehydration for salt formation and carboxylation: and for another, to prevent incomplete dehydration that results in the absence of carboxylation, a preliminary treatment under vacuum is necessary.
German Pat. No. 423034 describes an improved method wherein an alkali metal salt of .beta.-naphthol is reacted with carbon dioxide under pressure in the presence of excess .beta.-naphthol. The method is still unsatisfactory because .beta.-naphthol formed as an azeotropic product solidifies during dehydration, the yield of the end product is only about 70%, and the reaction requires a pressure as high as 40 to 100 kg/cm.sup.2 G.
Japanese Patent Application (OPI) Nos. 79256/79 and 79257/79 (the term "OPI" as used herein means an unexamined published Japanese Patent Application) describe another improved method wherein a liquid mixture comprising an alkali salt of .beta.-naphthol, .beta.-naphthol and gas oil or kerosine is reacted with carbon dioxide. The method also has several defects: dehydration is difficult to achieve in the heterogeneous liquid mixture that easily separates into two phases; a good suspension is particularly difficult to form in continuous reaction; and an additional apparatus is necessary to remove the excess solvent by separation.
Japanese Patent Application (OPI) No. 1723/72 proposes the use of diphenyl, diphenyl ether, or an alkyl naphthalene as the reaction solvent. The proposal is interesting in that the reaction with carbon dioxide is performed in a solution state, but it still has several defects: the solvent is expensive, it is either high-melting or low-boiling, and it has an unpleasant smell; and rapid dehydration is difficult.
As a result of various studies to eliminate the defects of the conventional techniques, it has been found that an object of the present invention can be achieved by first forming a homogeneous solution of an akali metal salt of .beta.-naphthol using .beta.-naphthol and an alkylbenzene of the formula (I) which is defined below and then reacting the solution with carbon dioxide.